Multiple RF return pad contact detection system

ABSTRACT

A multiple RF return pad contact detection system is provided which is adaptive to different physiological characteristics of patients without being susceptible to electrosurgical current interference (e.g., interference or measurement interaction between components of the detection system). The detection system can measure or sense the contact resistance or impedance between the patient and pairs of RF return pads or return electrodes where multiple pairs of RF return pads are utilized due to the high current frequently needed during electrosurgery while eliminating or minimizing the risk of measurement interaction between the RF return pad pairs. The system allows for the independent and simultaneous measurement of the pad contact impedance for each pair of RF return pads. If the impedance of any pad pair is above a predetermined limit, the system turns off or reduces the electrosurgical output of the electrosurgical generator to prevent excess heating. The system eliminates or minimizes interference or measurement interaction between the pad pairs by providing a different signal source frequency for each pad contact pair, but a frequency which matches an associated series resonant network frequency. The current that flows in the series resonant network is a direct reflection or function of the pad impedance of the corresponding pad pair.

BACKGROUND

1. Technical Field

The present disclosure is directed to electrosurgery and, in particular,to circuitry for measuring or sensing the contact resistance orimpedance between the patient and pairs of RF return pad contacts orelectrodes employed in such surgery.

2. Description of the Related Art

One potential risk involved in electrosurgery is the possibility ofstray electrical currents causing excess heating proximate the RF returnpad contacts or patient return electrodes. The most common conditionswhich are thought to lead to excess heating include:

(1) Tenting: Lifting of the return electrode from the patient due topatient movement or improper application. This situation may lead toexcess heating if the area of electrode-patient contact is significantlyreduced;

(2) Incorrect Application Site: Application of a return electrode over ahighly resistive body location (e.g., excessive adipose tissue, scartissue, erythema or lesions, excessive hair) will lead to a greater,more rapid temperature increase. Or, if the electrode is not applied tothe patient (i.e. electrode hangs freely or is attached to anothersurface), the current may seek an alternate return path such as thetable or monitoring electrodes; and

(3) Gel drying either due to premature opening of the electrode pouch oruse of an electrode which has exceeded the recommended shelf life.

Many monitor or detection systems have been developed in the past, butmost cannot directly guard against all three of the above listedsituations. In order to protect against these potentially hazardoussituations, the contact resistance or impedance between the returnelectrode and the patient should be monitored in addition to thecontinuity of the patient return circuit.

Safety circuitry is known whereby split (or double) patient electrodesare employed and a DC current (see German Pat. No. 1,139,927, publishedNov. 22, 1962) or an AC current (see U.S. Pat. Nos. 3,933,157 and4,200,104) is passed between the split electrodes to sense the contactresistance or impedance between the patient and the electrodes. U.S.Pat. No. 3,913,583 discloses circuitry for reducing the current passingthrough the patient depending upon the area of contact of the patientwith a solid, patient plate. A saturable reactor is included in theoutput circuit, the impedance of which varies depending upon the sensedimpedance of the contact area.

The above systems are subject to at least one or more of the followingshortcomings: (a) lack of sensitivity or adaptiveness to differentphysiological characteristics of patients and (b) susceptibility toelectrosurgical current interference when monitoring is continued duringelectrosurgical activation.

U.S. Pat. Nos. 4,416,276 and 4,416,277 describe a split-patient returnelectrode monitoring system which is adaptive to different physiologicalcharacteristics of patients, and a return electrode monitoring systemwhich has little, if any, susceptibility to electrosurgical currentinterference when monitoring is continued during electrosurgicalactivation. The entire contents of both U.S. Pat. Nos. 4,416,276 and4,416,277 are incorporated herein by reference.

Still a need exists for a detection or monitoring system, which is: 1)adaptive to different physiological characteristics of patients; 2) haslittle, if any, susceptibility to electrosurgical current interference,(including interference or measurement interaction between components ofthe detection system); 3) can measure or sense the contact resistance orimpedance between the patient and pairs of RF return pads or electrodeswhere multiple pairs of RF return pads are utilized due to the highcurrent frequently needed during electrosurgery, such as during tissueablation; and 4) eliminates or minimizes the risk of measurementinteraction between the RF return pad pairs.

Therefore, it is an aspect of the invention to provide a multiple RFreturn pad contact detection system for use during electrosurgicalactivation which achieves the above objectives.

SUMMARY

A multiple RF return pad contact detection system is disclosed which isadaptive to different physiological characteristics of patients, withoutbeing susceptible to electrosurgical current interference. The detectionsystem includes interference or measurement interaction betweencomponents of the detection system which can measure or sense thecontact resistance or impedance between the patient and pairs of RFreturn pads or electrodes when multiple pairs of RF return pads areutilized. Due to the high current frequently needed duringelectrosurgery, such as during tissue ablation, the detection systemeliminates or minimizes the risk of measurement interaction between theRF return pad pairs.

The circuitry of the multiple RF return pad contact detection system ispreferably provided within an electrosurgical generator for controllingthe generator according to various measurements, such as the contactresistance or impedance between the patient and pairs of RF return padsor return electrodes. The system allows for the independent andsimultaneous measurement of the pad contact impedance for each pair ofRF return pads. If the impedance of any pad pair is above apredetermined limit, the system turns off or reduces the electrosurgicaloutput of the electrosurgical generator to prevent excess heating.

The system eliminates or minimizes interference or measurementinteraction between the pad pairs by providing a different signal sourcefrequency for each pad contact pair, but a frequency which matches anassociated series resonant network frequency. The current that flows inthe series resonant network is a direct reflection or function of thepad impedance of the corresponding pad pair. Since the two resonantnetworks are tuned to different frequencies, there is minimalinteraction, if any, within the system, thus reducing the chances ofinaccurate measurements.

The system could be modified by providing a multiplexer to multiplex themeasurements corresponding to each pad contact pair to eliminate orminimize measurement interaction and also minimize hardware resources.

Further features of the multiple RF return pad contact detection systemof the invention will become more readily apparent to those skilled inthe art from the following detailed description of the apparatus takenin conjunction with the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention will be described herein below withreference to the drawings wherein:

FIG. 1 is a schematic diagram of the multiple RF return pad contactdetection system in accordance with a preferred embodiment of theinvention; and

FIG. 2 is a graph illustrating the operation of the pad contactimpedance measurement subsystem of FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Reference should be made to the drawings where like reference numeralsrefer to similar elements. Referring to FIG. 1, there is shown aschematic diagram of the multiple RF return pad contact detection system100 of the present invention wherein electrosurgical generator 10includes known circuitry such as a radio frequency oscillator 12 and anoutput amplifier 14 which generate an electrosurgical current. Thiscurrent is applied to a patient (not shown) via an active electrode 16.The electrosurgical current is returned to the generator 10 via padcontact pairs or return electrode pairs 18 a, 18 b having pads orelectrodes 20 a, 20 b and 22 a, 22 b and a corresponding two conductorpatient cable 24 a, 24 b having leads 26 and 28. Two capacitors 32 and34 are connected across each of the secondary windings 40 a, 40 b oftransformer 38 a, 38 b.

Each primary winding 36 a, 36 b is connected to a corresponding a.c.signal source 42 a, 42 b and a series resonant network 44 a, 44 b. Thepurpose of each series resonant network 44 a, 44 b is to produce acurrent (i.e., left and right current senses) which is a function of theimpedance between pads or electrodes 20 a, 20 b and 22 a, 22 b.

The system 100 eliminates or minimizes interference or measurementinteraction between the pads 20 a, 20 b and 22 a, 22 b, while allowingfor the independent and simultaneous measurement of the pad contactimpedance for each pair of RF return pads by having each a.c. signalsource 42 a, 42 b provide a different signal source frequency for itscorresponding pad contact pair. The frequency of each series resonantnetwork 44 a, 44 b is tuned to match the frequency of the currentproduced by its associated a.c. signal source 42 a, 42 b.

Accordingly, the frequency of one of the series resonant networks 44 ais different from the frequency of the other series resonant network 44b. Hence, there is minimal interaction, if any, between the left andright circuitry of the system 100, especially the two contact pad pairs18 a, 18 b. This essentially eliminates inaccurate or confusingmeasurements.

Additionally, the frequency of the electrosurgical current produced bythe electrosurgical generator 10 is substantially different from that ofthe current produced by the a.c. signal sources 42 a, 42 b.

The current that flows in each series resonant network 44 a, 44 b, i.e.,left and right current senses, is a direct reflection or function of thepad impedance of the corresponding pad contact pair 18 a, 18 b accordingto the physics of a series resonant network. Each series resonantnetwork 44 a, 44 b is an RCL network or a combination of R (resistance),L (inductance) and C (capacitance). In a preferred embodiment of theseries resonant networks 44 a, 44 b, the inductive component for eachnetwork is integrated into the respective transformer 38 a, 38 b.

The frequency response of a series resonant network has a maximumresonant frequency f_(R). At the resonant frequency, the series resonantnetwork has the minimum impedance, as opposed to a parallel resonantnetwork which has the maximum impedance at the resonant frequency, andthe phase angle is equal to zero degrees. The total impedance of aseries resonant network is Z_(T)+jX_(L)−jX_(C)=R+j(X_(L)−X_(C)). Atresonance: X_(L)=X_(C), f_(R)=1/(2πsqrtLC), Z_(T)=R, and V_(L)=V_(C).The resonance of a series resonant network occurs when the inductive andcapacitive reactances are equal in magnitude but cancel each otherbecause they are 180 degrees apart in phase.

The left and right current senses are applied to pad contact impedancemeasurement subsystem 46 which determines whether the impedancemeasurements between pads or return electrodes 20 a, 20 b and 22 a, 22 bare within a desired range. The range is preferably adaptable to thephysiological characteristics of the patient. If at least one of theimpedance measurements is not within a desired range, an inhibit signalis applied over a line 48 to internally disable the electrosurgicalgenerator 10 (or reduce the RF output therefrom) to prevent excessheating.

U.S. Pat. Nos. 4,416,276 and 4,416,277 describe a method for determiningthe desired range according to the physiological characteristics of thepatient, the entire contents of these patents is incorporated herein byreference.

Preferably, the desired range for which the impedance must fall betweenreturn electrodes 20 a, 20 b and 22 a, 22 b is about 20 to about 144ohms. If not, the electrosurgical generator 10 is disabled. Thus, in onemethod of operation of the present invention, the lower limit is fixedat the nominal value of 20 ohms, thus reducing the onset of patientinjury as a result of stray current paths which may surface if a contactpad or electrode is applied to a surface other than the patient. Theupper limit is set to avoid such problems as those mentionedhereinbefore, i.e., tenting, incorrect application site, gel drying,etc.

In accordance with an important aspect of the invention, the upper limitis adjustable from the absolute maximum (typically about 144 ohms)downward to as low as typically 20 ohms to thereby provide for automaticadaptiveness to the physiological characteristics of the patient. Thisprovides the multiple RF return pad contact detection system 100 of thepresent invention with significantly more control over the integrity ofthe RF pad contact or electrode connections without limiting the rangeof patient types with which the multiple RF return pad contact detectionsystem 100 may be used or burdening the operator with additionalconcerns.

That is, the physiological characteristics can vary significantly frompatient to patient and from one location site for the pad pairs toanother. Thus, patients may vary in their respective amounts of adiposetissue (which is one determining factor in the impedance measurementbetween the various pads) without effecting the detection system.Further, for a particular patient, one location site may be more fatty,hairy or scarred than another. Again, this does not reduce theeffectiveness of the system, i.e., all of these factors typically affectthe impedance measured between pads 20 a, 20 b and 22 a, 22 b and thusconcern the operator as to which site is optimal for a particularpatient. Such concerns are eliminated in accordance with the presentinvention by providing for automatic adaptability to the physiologicalcharacteristics of the patient.

Reference should now be made to FIG. 2 which is a graph illustrating theoperation of pad contact impedance measurement subsystem 46.

During operation, the desired impedance range (that is, the acceptablerange of the impedance detected between pads 20 a, 20 b and 22 a, 22 b)is preset when the power is turned on to an upper limit of, for example,120 ohms and a lower limit of, for example, 20 ohms as can be seen attime T=0 seconds in FIG. 2. If the monitored impedance for any padcontact pair is determined to be outside of this range (T=A seconds) bycomparing the current sense signal (or a signal derived there from) witha reference signal (e.g., a signal equal to 120 ohms or 20 ohms) usingcomparator circuitry (e.g., when a pad pair or any single contact pad isnot affixed to the patient) an alert will be asserted and theelectrosurgical generator 10 will be disabled over line 48.

The impedance between two contact pads of a contact pad pair at anyinstant is designated the return RF electrode monitor (REM)Instantaneous Value (RIV) in FIG. 2. When the REM impedance enters therange (T=B seconds) bounded by the Upper Limit (UL) and the Lower Limit(LL), a timing sequence begins. If after five seconds the RIV is stillwithin range (T=C seconds), the alert condition will cease and the REMimpedance value is stored in memory. This is designated as REM NominalValue (RNV). The upper limit is then reestablished as 120% of thisamount. The 80 ohm RIV shown in FIG. 2 causes the upper limit to be at96 ohms. This feature of the invention is particularly important becauseit is at this time (T=C seconds) that adaptation is initially made tothe physiological characteristics of the patient. Note if the RIV wereto exceed 96 ohms at a time between T=C and T=F seconds (while the upperlimit is 96 ohms), the alert will be asserted and the electrosurgicalgenerator 10 disabled.

However, if the upper limit had not been adjusted to 96 ohms, the alertwould not have been asserted until after the RIV exceeded the initial120 ohms upper limit as determined by the comparator circuitry, thuspossibly heating one or both of the pads 20 a, 20 b and 22 a, 22 b. Thissituation is of course exacerbated if the patient's initial RIV withinthe preset 20 to 120 ohm range is 30 ohms.

An initial RIV of 10 ohms within the preset range of 20 to 120 ohms setsan upper limit of 144 ohms.

In accordance with another aspect of the invention, it has been observedthat the impedance between contact pads of contact pad pairs decreasesover a relatively long period, such as a number of hours. Since manysurgical procedures can extend a number of hours, this effect is alsotaken into consideration in the present invention. Accordingly, RIV iscontinuously monitored and any minima in REM impedance (e.g., a downwardtrend followed by a constant or upward trend in REM impedance) initiatesa new five second timing interval (T=E seconds) at the end of which theRNV is updated to the RIV if the RIV is lower (T=F seconds). The REMupper limit of 120% of RNV is re-established at this time. The fivesecond interval causes any temporary negative change in REM impedance(T=D seconds) to be disregarded. Operation will continue in this mannerprovided RNV does not exceed the upper limit of 120% RNV or drop belowthe lower limit of 20 ohms. Exceeding the upper limit (T=G seconds)causes an alert and the electrosurgical generator 10 is disabled. Itwill remain in alert until the RIV drops to 115% of RNV or less (T=Hseconds) or until the system 100 is reinitialized. RIV dropping to lessthan 20 ohms (T=I seconds) causes a similar alert which continues untileither the RIV exceeds 24 ohms (T=J seconds) or the system 100 isreinitialized. The hysteresis in the limits of the REM range (that is,the changing of the upper limit to 115% of RNV and the lower limit to 24ohms in the previous examples) prevents erratic alerting when RIV ismarginal.

It should be noted in the example of FIG. 2 that the alert actually doesnot turn off when RIV returns to a value greater than 24 ohms becausethe pad pairs are removed before 5 seconds after T=J seconds elapse.Thus, the alarm stays on due to the removal of the pad contact pairs 18a, 18 b.

Removing the pad contact pairs 18 a, 18 b from the patient or unpluggingthe cables 26, 28 from the electrosurgical generator 10 (T=K seconds)for more than one second causes the system 100 to be reinitialized tothe original limits of 120 and 20 ohms. This permits a pad to berelocated or replaced (T=L seconds) without switching theelectrosurgical generator 10 off. The RIV at the new location is 110ohms and 120% RNV is 132 ohms. Thus, as described above, this is the onetime (whenever RIV enters the 20 to 120 ohms range (either as presetduring power on or as reinitialized as at T=K seconds) for the firsttime) that the upper limit can be raised during the normal REM cycle.Otherwise, it is continually decreased to adapt to the decreasing RIVimpedance with the passage of time.

The preferred implementation of the foregoing FIG. 2 operation of thepad contact impedance measurement subsystem 46 is effected by a set ofprogrammable instructions configured for execution by a microprocessor.

The system 100 could be modified by providing a multiplexer to multiplexthe measurements corresponding to each pad contact pair 18 a, 18 b toeliminate or minimize measurement interaction and also minimize hardwareresources.

Other pad contact pair arrangements can be provided in the system 100 ofthe present invention besides the pad pair arrangements shown in FIG. 1.For example, ten pad contact pairs 18 can be provided and connected toelectrosurgical generator 10 by cables 26 and 28, where thecorresponding a.c. signal source 42 and series resonant network 44corresponding to each pad contact pair 18 are tuned to the samefrequency which is different from the frequency of the other a.c. signalsources 42 and series resonant networks 44.

It is provided that the system 100 of the present invention allows forimpedance comparisons to be performed between pad pairs. Therefore, ifthe pad pairs are placed symmetrically on the patient, i.e., left legand right leg, comparison of the contact impedance can provide anotherdegree of detection and safety.

Although the subject apparatus has been described with respect topreferred embodiments, it will be readily apparent to those havingordinary skill in the art to which it appertains that changes andmodifications may be made thereto without departing from the spirit orscope of the subject apparatus.

1-22. (canceled)
 23. A method for adaptive impedance monitoring of atleast one patient return pad configured for contacting a patient andtransmitting electrosurgical energy back to an electrosurgicalgenerator, the method comprising the steps of: selecting a desiredimpedance range having a lower limit and an upper limit; recordingimpedance of at least one patient return pad to obtain an instantaneousimpedance value; determining whether the instantaneous impedance iswithin the impedance range; updating the upper limit as a function ofthe instantaneous impedance value according to the determination of thedetermining step; and monitoring the impedance of the at least onepatient return pad to determine if the impedance is between the lowerlimit and the updated upper limit.
 24. A method according to claim 23,further comprising the step of measuring initial impedance of the atleast one patient return pad to determine whether the initial impedanceis within the desired impedance for a predetermined interval of time.25. A method according to claim 24, further comprising the step ofgenerating a control signal for controlling the operation of anelectrosurgical generator according the determination made by themonitoring impedance step.
 26. A method according to claim 25, whereinthe control signal of the generating step signals the electrosurgicalgenerator to perform an operation selected from the group consisting ofissuing an alert and adjusting supply of electrosurgical energy.
 27. Amethod according to claim 23, wherein the function of the updating stepfor updating the upper limit is multiplication of the instantaneousimpedance by a factor larger than 1.0.
 28. A method according to claim23, further comprising the step of detecting a termination of a downwardimpedance trend to determine whether the instantaneous impedance valueis a minimum impedance value.
 29. A method according to claim 28,further comprising the step of updating the upper limit as a function ofthe instantaneous impedance value according to the determination of thedetecting step.
 30. An electrosurgical system comprising: anelectrosurgical generator configured to generate electrosurgical energy,the electrosurgical generator coupled to least one patient return padconfigured for contacting a patient and transmitting electrosurgicalenergy back to the electrosurgical generator; an impedance measurementsubsystem coupled to the at least one patient return pad and adapted torecord impedance of at least one patient return pad to obtain aninstantaneous impedance value; and a microprocessor configured to:select a desired impedance range having a lower limit and an upperlimit; update the upper limit as a function of the instantaneousimpedance value according to the determination whether the instantaneousimpedance is within the impedance range; and monitor the impedance ofthe at least one patient return pad to determine if the impedance isbetween the lower limit and the updated upper limit.
 31. Anelectrosurgical system according to claim 30, wherein the impedancemeasurement subsystem measures initial impedance of the at least onepatient return pad and the microprocessor is further configured todetermine whether the initial impedance is within the desired impedancefor a predetermined interval of time.
 32. An electrosurgical systemaccording to claim 31, wherein the microprocessor is configured togenerate a control signal which controls the operation of theelectrosurgical generator according the determination made by themonitoring impedance step.
 33. An electrosurgical system according toclaim 32, wherein the control signal of the microprocessor signals theelectrosurgical generator to perform an operation selected from thegroup consisting of issuing an alert and adjusting the supply ofelectrosurgical energy.
 34. An electrosurgical system according to claim30, wherein the upper limit is updated by multiplying the instantaneousimpedance by a factor larger than 1.0.
 35. An electrosurgical systemaccording to claim 30, wherein the microprocessor is configured todetect a termination of a downward impedance trend to determine whetherthe instantaneous impedance value is a minimum impedance value.
 36. Anelectrosurgical system according to claim 35, wherein the microprocessoris configured to update the upper limit as a function of theinstantaneous impedance value if the instantaneous impedance value isthe minimum impedance value.